The present invention relates to processing photodetector signals, particularly photodetector signals from photodetectors in arrays of photodetectors which are used as image plane staring sensors and where there is a large background component in the image. More particularly, the signal processor of the present invention is especially used in connection with infrared staring sensors.
The use of charge-transfer device technology to process signals obtained from photodetectors has a number of advantages. First, charge-transfer devices, particularly charge-coupled devices, can be relatively easily fabricated in silicon monolithic integrated circuits and fabricated such that these devices individually are capable of being provided therein in a high density. The charge-transfer device with respect to analog signals, is basically a sampling device directly manipulating the analog samples. Thus, the interface between such a device and the photodetector can be relatively uncomplicated since the photodetector, typically, provides an analog electrical output signal more or less related to that electromagnetic energy which is to be sensed. Further, the analog samples to be manipulated in the charge-transfer device can be controlled by digital clocking circuits which permit considerable flexibility in treating the analog samples.
However, there are difficulties in processing photodetector output signals, particularly infrared photodetector output signals. In processing these signals, there is the problem that the signals reflect the low contrast generally occurring in infrared scene image coupled with a very large background radiation portion also occurring therein. This situation leads to changing signals representing scene image details and changes, which are relatively small because of the low contrast, coupled with a large and relatively constant signal component due to the large background portion. As a result, direct coupled staring sensors have difficulty distinguishing between variations in detector responsivity among photodetectors present within the array versus actual variations in image irradiance. Thus, there is a stringent limitation on the variance among detector responsivities in such a system if other measures are not taken.
A typical arrangement is to use a two-dimensional photodetector array in a mechanically scanned arrangement. This arrangement is used with the time delay and integration (TDI) signal processing method to improve the signal strength relative to the noise, coupled with use of capacitive coupling (AC coupling) between the photodetectors and charge-coupled device signal processing circuitry to remove relatively constant signal portions.
Alternatively, to avoid scanning, staring sensors are used with capacitive coupling between the photodetectors and a charge-coupled device processing structure to thereby eliminate the steady background signal, the latter structure being used as a multiplexor. Often with, or as an alternative to capacitive coupling, a substantial amount of analog or digital processing is required at the output of the charge-coupled device signal processor to provide signal enhancement. This processing at the output may leave the charge-coupled device, in handling the background signal component in each sample, requiring in its structure rather large sample storage well electrodes. A typical processing method is to subtract successive scene frame values from one another so that only the changes are kept, frames being defined as time periods of successive "snapshots" of the image scene in time, i.e., as times of successive samples of the scene image. Another alternative is to subtract from a frame value a "blanked" frame value representing the background level, or to subtract therefrom a reference level either derived from the scene in some other way or somehow otherwise estimated. A method to reduce the required digital processing, and which can reduce the size of the storage well electrodes in the charge-coupled device in a monolithic integrated circuit, is to use a charge skimming technique where a portion of the sample of the image scene corresponding to the background is removed with each sampling.
Another problem in the use of charge-transfer devices in signal processors for infrared photodetectors is the problem of bright spots occurring in the image scene. The result is "blooming" or the overflow of charge, representing a sample of a bright spot in the scene viewed, from storage wells holding that sample into adjacent storage wells. The result is the obliterating of the representation of other portions of the scene associated with the adjacent storage well, and so the distorting of a substantial portion of the image scene. A number of special purpose circuits have been devised for controlling such "blooming".
There is a substantial need for a better and more efficient signal processor for use with a staring sensor, thereby avoiding the need for mechanical scanning, which can also overcome the problems of low contrast and "blooming". Further, such a signal processor is desired to be as small as possible to promote the greatest density of photodetector elements in the image array associated with the processor where this array is to be provided at the focal plane of a system receiving electromagnetic energy from which to form an image.